1. Field of the Invention
The present invention relates to a magnetic recording medium. More specifically, the present invention relates to a magnetic recording medium which is excellent in surface properties and capable of achieving high output.
2. Description of the Related Art
Magnetic layers comprising a ferromagnetic iron oxide, a Co-modified iron oxide, CrO2, or a ferromagnetic alloy powder dispersed in a binder, which are coated on a nonmagnetic support, have been widely known in magnetic recording media such as a video tape, an audio tape, a magnetic disc, etc. In recent years, recording wavelength tends to become shorter with the increase of density of recording system. The thickness loss at recording/reproduction, such as the reduction of output, has become a problem in a magnetic layer having a comparatively thick layer thickness. Therefore, the thickness of a magnetic layer has been thinner. As a method of thinning a magnetic layer while maintaining surface properties, a method of simultaneous multilayer coating comprising coating an upper thin magnetic layer on a lower nonmagnetic layer has been adopted.
However, there are problems in such a simultaneous multilayer coating method such as noise generates due to the turbulence in the interface between the upper layer and the lower layer and the packing density of the upper magnetic layer reduces due to the migration of the binder from the lower layer to the upper layer (the binder contained in the lower layer migrates to the upper layer where the concentration of the binder is lower than that of the lower layer). As a result, good electromagnetic characteristics cannot be obtained. For resolving these problems, based on the thought that the reduction of the numbers of interfaces resulting in the reduction of noises, a magnetic recording medium comprising a single magnetic layer without providing a lower nonmagnetic layer was produced and evaluation was carried out. However, contrary to our expectations, electromagnetic characteristics could not be improved in the case of a thin layer medium having the dry thickness of a magnetic layer of 0.31 xcexcm or less.
It is an object of the present invention to provide a magnetic recording medium having an excellent surface property and an excellent electromagnetic conversion characteristic.
As a result of the inventors"" examination, it was found that surface properties of a magnetic layer were deteriorated due to the presence of nonmagnetic particles such as abrasive particles or carbon particles contained in a magnetic solution for forming a magnetic layer, thereby desired electromagnetic characteristics could not be obtained.
As a result of further examinations, the present inventors have found that a magnetic recording medium having good surface properties and excellent electromagnetic conversion characteristics could be obtained even in a thin layer medium having the dry thickness of a magnetic layer of 0.3 xcexcm or less by using nonmagnetic particles having the relationship that the dry thickness of the magnetic layer is equal to or larger than the average particle diameter of the nonmagnetic particles. Thus, the present invention has been attained.
According to the present invention, a magnetic recording medium comprising a support and a single magnetic layer having a dry thickness of 0.3 xcexc or less (preferably, 0.01 xcexcm or more) provided on the support, wherein the average particle diameter of the nonmagnetic particles contained in the magnetic solution comprising the magnetic layer and the dry thickness of the magnetic layer have a relationship where the dry thickness of the magnetic layer is equal to or larger than the average particle diameter of the nonmagnetic particles.
When a magnetic recording medium has a magnetic layer having a dry thickness of 0.3 xcexcm or less (preferably, 0.01 xcexcm or more) and a nonmagnetic lower layer is provided under the magnetic layer, even if nonmagnetic particles larger than the thickness of the magnetic layer are present in the magnetic layer, they are crushed to the lower nonmagnetic layer by calendering process. As a result good surface properties can be obtained.
However, when a magnetic recording medium comprises a single magnetic layer having a dry thickness of 0.3 xcexcm or less (preferably, 0.01 xcexcm or more), nonmagnetic particles in a magnetic layer cannot be crushed to the lower nonmagnetic layer by calendering process, therefore, the surface properties are deteriorated.
Accordingly, it has been found that for ensuring surface properties it is necessary to use nonmagnetic particles which satisfy the relationship where the dry thickness of a magnetic layer is equal to or larger than the average particle diameter of the nonmagnetic particles.
As a result of further investigations, when a support satisfying the condition that, when measured using ATM, in 30 xcexcmxc3x9730 xcexcm square, the support has 1,000 or less protrusions having a height of 30 nm or more, 500 or less protrusions having a height of 40 nm or more, and 200 or less protrusions having a height of 50 nm or more is used, good surface properties and excellent electromagnetic characteristics can be obtained in a thin magnetic medium having a dry thickness of 0.3 xcexcm or less (preferably, 0.01 xcexcm or more). Thus, the present invention has been achieved.
That is, the above object of the present invention has been resolved by a magnetic recording medium comprising a support and a single magnetic layer having a dry thickness of 0.3 xcexcm or less (preferably, 0.01 xcexcm or more) provided on the support, wherein the surface of the support has 1,000 or less protrusions having a height of 30 nm or more, 500 or less protrusions having a height of 40 nm or more, and 200 or less protrusions having a height of 50 nm or more when measured using ATM in 30 xcexcmxc3x9730 xcexcm square.
Here, the magnetic layer defined in the present invention means a magnetic recording layer which can perform recording by magnetism.
Further, the single layer defined, in the present invention includes not only a single magnetic layer medium but also the case comprising a magnetic layer medium and a thin undercoating layer for improving the adhesion property of the magnetic layer medium, or a thin overcoat lubricating layer for improving running property and durability of the magnetic recording layer.
The above-described nonmagnetic particles include abrasive particles and carbon particles.